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In the 1980s, researchers studying the effects of acidic deposition on forests reported that anthropogenic emissions of nitrogen-containing compounds could adversely affect forest soils and vegetation. These effects were similar to those caused by sulfur-containing compounds in the eastern U.S. That is, chronic nitrogen deposition could accelerate soil acidification, deplete levels of essential alkaline cations, and reduce tree growth. Because forest soils are typically nitrogen-deficient, it was hypothesized that after several decades of high nitrogen deposition, nitrogen levels in selected forest soils would be present in excess of that needed to sustain healthy rates of tree growth. Once forest soils became nitrogen-saturated, a set of responses indicative of forest deterioration caused specifically by excess nitrogen would become evident. In addition to loss of alkaline cations and nitrate to deep layers of soil and groundwater, elevated levels of nitrate in streams draining from affected forests and high rates of nitric oxide emissions from soil would be detectable. Evidence of these responses would indicate that the forest had changed from being nitrogen-deficient to being nitrogen-saturated.
In consideration of findings from a project conducted at Barton Flats during the period 1991-1994, questions were raised as to the total amount of nitrogen being deposited to forests at the more polluted western end of the SBM. At Barton Flats it was found that pine trees were exposed to harmful concentrations of ozone (daytime 12-hour average ~ 0.06 ppm) and moderate levels of atmospheric nitrogen deposition (5 to 9 kg/ha/yr). Much higher exposures to both pollutants were expected to occur at the western end of the SBM, as well as more severe tree injury from ozone and adverse effects on soil nutrient cycling. This project was conducted to determine whether forest sites located at the polluted end of an ozone and atmospheric nitrogen exposure gradient in the SBM exhibited signs of nitrogen saturation. The field work was initiated in winter 1995 and concluded in winter 1997. |
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Fog was collected at Camp Paivika and Barton Flats at weekly intervals to characterize fog chemistry at a western, high deposition site, and an eastern, moderate-to-low deposition site, respectively, in the SBM. Throughfall was collected on an event basis using bulk deposition collectors at Camp Paivika and Barton Flats. Stream water samples were collected monthly at 19 sites to assess trends in nitrate concentrations. Seven streams were located in Devil Canyon near Camp Paivika, and twelve sites were in the San Gorgonio Wilderness near Barton Flats. Within the San Gorgonio Wilderness, samples were collected at five southwestern sites (moderate nitrogen deposition), and at seven northern sites (low nitrogen deposition). Soil nitric oxide emissions were measured monthly at Camp Paivika and Camp Osceola, a low nitrogen deposition site about 4 km east of Barton Flats. |
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In fog, peak nitrogen concentrations were 4 to 5 times higher, and annual rates of nitrogen deposition were 13 to 14 times greater at Camp Paivika than at Barton Flats. At these rates of deposition, fog is an important source of nitrogen to forests at the western end of the SBM, but not at the eastern end.
Throughfall volume was 32 times greater at Camp Paivika than at Barton Flats during the sampling period. In addition, concentrations of nitrate and ammonium were 4 and 12 times higher, respectively, at Camp Paivika than at Barton Flats. Annual rates of total nitrogen deposition were estimated to be ~32 kg/ha/yr at Camp Paivika, similar to rates reported at sites where nitrogen saturation has occurred. (Note: total nitrogen deposition at Barton Flats was estimated to be ~5 kg/ha/yr.)
Consistent with expected differences in atmospheric nitrogen deposition, streams draining Devil Canyon had a higher median nitrate level (61 µeq/L) than either subgroup of sites in the San Gorgonio Wilderness. The difference in stream water nitrate levels between Devil Canyon and the San Gorgonio Wilderness was due in part to higher nitrate levels in springs at the respective sites, which have received vastly different amounts of atmospheric nitrogen over the last 30-40 years. In the San Gorgonio Wilderness, the five southwestern streams exhibited a higher median nitrate level than the seven northern streams (19 vs. 0.2 µeq/L).
Emissions of nitric oxide were substantially higher at Camp Paivika than at Camp Osceola, located about 4 km east of Barton Flats. The investigators estimated that the annual nitric oxide emissions from Camp Paivika were 4.5 kg N/ha/yr and those at Camp Osceola 0.25 kg N/ha/yr. |
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Throughout the SBM and other forested areas in southern California, pine trees are exposed to harmful levels of ozone and nitrogenous pollutants throughout the growing season. Previous studies conducted at Barton Flats indicated that soils at the site were not nitrogen-saturated, but may be at sites to the west, where exposures to ozone and nitrogenous pollutants are much greater. The findings from this study provide evidence of nitrogen saturation at Camp Paivika, a site that is 42 km west of Barton Flats. It is estimated that nitrogen losses due to stream water runoff and soil emissions are of similar magnitude to inputs from atmospheric deposition, a clear sign that the soil at Camp Paivika is nitrogen-saturated. The long term outlook for pine forests at the western end of SBM is not promising unless major reductions in ozone exposure and nitrogen deposition are achieved. |
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Previous ARB-sponsored research projects on the effects of air pollutants on ponderosa pine include controlled studies conducted at the U.S. Forest Service Tree Improvement Center in Chico, California, Shirley Meadow in the southern Sierra Nevada, and Barton Flats in the SBM. ARB contract numbers are given in brackets. At Chico, branch exposure chambers were used to investigate the effects of acidic deposition and ozone on gas exchange responses in mature branches and seedlings of ponderosa pine (Gas Exchange by Pinus ponderosa in Relation to Atmospheric Pollutants [A132-101] and Determination of Acidic Gas and Particle Concentrations in Open-Top Field Chambers [A132-174]). At Shirley Meadow, the growth and physiological responses of ponderosa pine seedlings exposed to one or two years of ozone were evaluated (Growth, Physiological and Biochemical Responses of Ponderosa Pine (Pinus ponderosa) to Ozone [A733-137, A833-083, and A033-056]). At Barton Flats, a three-year study was conducted to measure ozone concentrations, estimate rates of nitrogen deposition, and assess pine tree health and soil quality (Assessment of Acidic Deposition and Ozone Effects on Conifer Forests in the San Bernardino Mountains [A032-180] and Ecosystem-Level Alterations in Soil Nutrient Cycling: An Integrated Measure of Cumulative Effects of Acidic Deposition on a Mixed Conifer Forest in Southern California [92-335]).
To further examine the long-term effects of air pollution on pine tree health, a two-year study to identify biochemical changes in pine tree wood will be conducted by a team of researchers from the University of California, Davis, and the U.S. Forest Service. Tree cores will be collected from forests along known gradients of ambient ozone exposure in southern California and the Sierra Nevada and analyzed using pyrolysis/gas-chromatography/mass-spectrometry and Fourier-transform infrared microspectroscopy. The study, which will be completed in July 2000, is entitled Historical-Scale Biochemical Markers of Oxidant Injury and Exposure in Pines, ARB contract number 97-309. |
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